This invention relates to an alloy for a dental amalgam and, more particularly, to a new class of such alloys which include silver, tin and manganese.
Amalgams are presently the principal material used by dentists for restoration of decayed teeth. About 75% of dental restorations are by amalgams. Amalgams are plastic at normal room and body temperature for a few minutes before they harden. Little or no change in volume occurs as a result of becoming hard. Amalgams combine the characteristics of high compressive and moderate tensile strength with the ability to withstand the corrosive environment defined by the mouth. Additionally, they are substantially non-toxic.
Generally, the alloy from which amalgams are made includes a mixture of silver and tin. The American Dental Association has established various standards for such alloys. Following is the American Dental Association specification for compositions of alloys used in making amalgams:
______________________________________ Silver Tin Copper Zinc Mercury Min wt Max wt Max wt Max wt Max wt % % % % % 65 29 6 2 3 ______________________________________
The above composition standard was adopted by the American Dental Association effective June 1, 1970 and is also identified as American National Standard No. Z156.1-1970. Incorporated herewith by reference is the publication entitled "Guide to Dental Materials and Devices", Seventh Edition 1974- 1975, copyright 1974, American Dental Association. Particular attention is directed to chapter 3 of this reference entitled "Amalgam and Mercury" as well as Specification No. 1 of the A.D.A. specifications for dental materials.
Amalgam alloys complying with present specifications and standards are generally silver-tin alloys containing approximately three parts of silver and one part of tin. This alloy is often referred to as the gamma phase (.gamma.) or Ag.sub.3 Sn. In practice, the powdered alloy and mercury are subjected to trituration, thereby facilitating a reaction between mercury and the alloy. The mercury combines with the alloy to form new solid phases from the pulverized and triturated amalgam.
The chemical reaction during amalgamation may be described as follows: EQU Ag.sub.3 Sn+Hg.fwdarw. Ag.sub.2 Hg.sub.3 + Sn.sub.7 Hg
Thus, in addition to a gamma.sub.1 phase (.gamma..sub.1) (Ag.sub.2 Hg.sub.3), a tin-mercury phase, often referred to as the gamma.sub.2 phase (.gamma..sub.2), is formed. The gamma.sub.2 phase has a simple hexagonal crystal structure and may contain 5 to 12% atomic percent mercury. The composition for this phase is uncertain; though, the phase is often designated as Sn.sub.7 Hg or Sn.sub.8 Hg.
The tin-mercury phase in a dental amalgam is known as a weak constituent relative to the silver-tin and silver-mercury phases. Nonetheless, the gamma.sub.2 phase may comprise up to 10% of the amalgam. The gamma.sub.2 phase has been associated with poor corrosion resistance and excessive flow or creep under an applid stress. To overcome the deficiencies noted in the gamma.sub.2 phase, the subject matter of the present invention has been developed.